Catchments

Everybody lives in a catchment.

A catchment can be described as an area of land, bounded by hills or mountains from which surface and groundwater flow into streams, then join and ultimately have the same outlet to the sea.

Catchments in New Zealand vary in size from large, such as the Waikato River stretching from Taupo to Pukekohe, to tiny areas of only a few hectares.

A lot of the urban waterways in Auckland have catchments only several hectares in area. Many of these waterways are less than 3 kilometres in length and are small enough that you could easily jump across them during low flows! We have about 9,500 kilometres of these types of streams in the region, about 90% of the total.

Large catchments are also made up of many smaller sub-catchments. Whatever happens in the smaller feeder streams affects the overall wellbeing of the main waterway lower down. Streams are generally described as permanently flowing (perennial) or intermittently flowing (ephemeral) which means they dry up in summer.

Tangata Whenua understand their ancestral waterways in terms of tribal boundaries and relationships. These tribal areas are identified by key geographic features such as maunga (mountains), awa (rivers) and puna (water sources/springs). These form the basis of Iwi and Hapu identity and spiritual and physical sustenance.

How catchments affect streams

Catchments vary along their length naturally as a result of gradient change. Gradient refers to the steepness of the stream bed. Generally we can split a stream into three sections – the upper, middle and lower catchment. Here are some details about the characteristics of each section.

Type

Channel Shape

Gradient

Substrate

Upper Catchment

V

Steep gradient and pools

Boulders, cobbles

Middle Catchment

U

Moderate gradient riffles and pools

Boudlers, cobbles, gravel

Lower Catchment

Wide

Low gradient, meandering

Gravel, silt

Why focus on catchments?

When we refer to the catchment scale, it means that we are looking at more than just the little bit of stream in front of us referred to as the stream reach. There are also tributaries, which feed into the main stream which eventually enters the sea.

By looking at things at a larger scale, we can see patterns and processes that occur over whole systems. For example, weed management within the riparian margin. If your group is controlling weeds near your monitoring site, you might find it quite frustrating that the weeds keep reappearing. Chances are, that upstream there are lots of weeds which will wash down and turn up to spoil your hard work. This is why looking at the big picture is so important.